A chain saw includes three basic components: a powerhead, a guide bar, and a cutting chain. The power head includes a motor that drives a sprocket. The guide bar attaches to and extends outwardly from the sprocket of the power head. The chain encircles the guide bar and the sprocket engages and moves the chain about the guide bar for cutting operation.
The guide bar is typically an oval-shaped metal plate defining along its edge an oval path for the cutting chain. The plate or bar is typically about 0.150 inch thick and the path around the bar edge is characterized by a center groove of about 0.050 inch width flanked by side rails also of about 0.050 inch width. The saw chain is made up of side links and center links. Tang portions extend from the center links into the groove and ride in the groove to assure entrainment of the saw chain around the guide bar periphery. The side links rest against the top of the side rails.
Early versions of the guide bar were produced from a single thickness metal plate, e.g., a 0.150 inch thick plate with the center groove around the bar edge being machine cut. The metal plate material and the process of cutting the groove are high-cost items and have been for many years the target of development efforts to reduce the overall cost of a chain saw. A secondary factor concerns the weight of the chain saw and the solid steel bar contributes substantially to that weight.
Out of these development efforts came the laminated saw chain bar. Using the above example, it will be appreciated that two 0.050 inch thick outer side laminates and a 0.050 inch center or core laminate, properly configured and assembled, will produce the oval-shaped bar with an edge groove. The total materials cost is somewhat reduced but, more importantly, the expensive groove cutting operation is eliminated. Added to the production cost, however, is the cost of fastening the laminates together, typically by spot welding.
It has long been recognized that the center or core laminate does not require the high strength characteristics of steel. Numerous proposals have been made to replace the core with a lighter, less expensive material. Examples of such developments are described in U.S. Pat. Nos. 3,473,581 (Merz), 3,191,646 (Merz), 4,693,007 (Apfei), and 4,383,590 (Pantzar).
These prior proposals reduce weight and materials cost, but add new problems that have generally resulted in as high or higher total production cost. Fastening the outer laminates in the appropriate relative spacing is a major consideration. Also, not previously discussed, is the desirability of forming the grooves so that the oil for lubrication can be injected into the groove and available for the various sliding components, i.e., the side links sliding on the top of the side rails and tang portions sliding along the sides of the bar groove.
U.S. Pat. No. 3,416,578, issued Dec. 17, 1968 to F. T. Irgens, proposes a reduced weight chain saw bar having outer side plates separated by a spacer plate. The spacer plate is perforated to reduce overall saw bar weight. A channel member is mounted intermediate the side plates and about the periphery of the structure to define a saw chain groove. The width of the spacer plate and channel member cooperatively define the relative spacing of the outer side plates.
A recent advance in the production of light weight chain saw bars is the foam core saw chain bar described in U.S. Pat. No. 4,885,843 issued Dec. 12, 1989 to Kelsay, III et al and assigned to the assignee of the present invention. The entire disclosure of U.S. Pat. No. 4,885,843 is incorporated herein by reference. Production of the described foam core bar requires placement of a bifurcated core forming plate between the outer laminates to define an adhesive receiving space therebetween. The receiving space extends to near the edge boundary of the outer laminates, but is spaced inwardly from the edge boundary by a distance corresponding to a desired groove depth. After the foam core material cures, the forming plate must be removed in a final production step. A saw chain groove about the periphery of the chain saw bar remains with the exposed foam core defining the bottom of the saw chain groove.